The index, which tracks data since 1985, shows that economic insecurity has risen across all groups, not just among low-income families and those without much education.

The index was constructed by Yale political scientist Jacob Hacker and a team of researchers, and the project was funded by the Rockefeller Foundation.

The ESI defines people as economically insecure when their situation meets two criteria. First, within a year's time they have lost 25% or more of their available gross income. Available gross income is the money they have left over after paying for medical costs and debt.Second, they don't have enough in an emergency fund or other liquid reserves to make up the difference.

Hacker noted that it can typically take between six to eight years to restore one's available income to its previous level. Meanwhile, a survey cited by Hacker found that 48% of Americans said last year they only had enough resources to carry them for two months before experiencing any economic hardship.

According to the index,which is based primarily on Census Bureau data,12.2% of Americans were economically insecure in 1985. By 2009, Hacker and his team estimate that 20.4% of Americans could be classified that way. The actual number of people affected increased by more than half, from 28 million in 1985 to roughly 46 million by 2007, the last year for which hard numbers were available.

"There's been a ratcheting up over time," Hacker said.

'Virtually all' groups affected

A large loss in income can occur for any of several reasons. The loss of a job is the most obvious. But even if one remains employed, income can vary because of a change in hours or wages, or because of a spike in medical expenses or debt payments.

While there have been intermittent years when the ESI fell for any given demographic group, economic insecurity has been on the rise for "virtually all" groups, according to the researchers.

But not all groups have equal levels of risk. The groups showing the highest levels of economic insecurity include African-Americans, the poorest households, Americans between the ages of 18 and 34, single-parent households and Americans without high school educations.

Since it has just been launched, the Economic Security Index has yet to be publicly critiqued by other economists and political scientists unaffiliated with the project.

In the past, some economists, such as Stephen Rose of the moderate-progressive think tank The Third Way, have conducted research that counters the broadly negative view about how the middle class has fared economically over the years.

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